Part of the inflammatory process is accomplished by the circulating white blood cells, especially neutrophils, by (1) slowing down at a site within the blood vessel where an inflammatory response is developing, (2) adhering to the endothelial cells adjacent to the site of inflammation, (3) exiting the intravascular space through the lining of the blood vessels and (4) migrating to the inflammatory site within the tissues. In order to achieve this complex series of steps, the neutrophil must bind to the endothelial cells lining the vessels and then "unbind" so that it can continue on into the tissue. It is known that sialic acid residues on glycoconjugates of the cells are important to this initial binding.
The term "sialic acid" denotes members of a family comprising natural derivatives of neuraminic acid, an acid amino pyranose with 9 carbon atoms. In nature, the amino group is substituted either with an acetyl or glycolyl residue. The hydroxy groups may be methylated or esterified with groups such as acetyl, lactyl, sulfate, or phosphate groups. Multiple substitutions are common.
Sialic acids are a phylogenetically conserved family. These amino sugars are conjugated to protein and lipid moieties on the surface of mammalian cells and are potent modulators of biologic behavior. There is substantial evidence that sialic acids are structural determinants of important cell-to-cell interactions and cellular functions such as adhesiveness. There is considerable evidence that sialic acid residues protect molecules in circulation from recognition, clearance or degradation and that they regulate complement deposition on cell surfaces. Sialic acid residues also modulate attachment of microbial toxins as well as parasites to these cell surfaces.
The cleavage of the sialic acid by sialidases or neuraminidases from the glycoconjugates results in decreased rigidity of the cell surface, thereby facilitating cell motility, and effects cell-to-cell interactions such as adhesiveness and metastatic potential. Sialidases or neuraminidases are produced by many microbes and by mammalian cells. Whereas the presence of endogenous sialidase of mammalian cells has been well described, its role has best been studied primarily in a clinically heterogenous group of inherited disorders designated as sialidoses, wherein an abnormal amount of sialic acid accumulates in tissues of patients resulting in neurologic defects and premature death.
Endogenous sialidase in phagocytes has previously been described. It has been found that, upon activation such as may occur during infection or inflammation, this enzyme is translocated to the cell surface from sites within the cell (Cross and Wright, Journal of Clinical Investigation. Inc., 88 (December, 1991) pp 2067-2076). The result of this mobilization is the removal of significant quantities of cell-associated sialic acid from glycoconjugates on cell surfaces. Desialylation of resting cells by microbial neuraminidase or of activated cells by mobilization of endogenous sialidases remove negative electric charges from cell surfaces and alters the biologic behavior of these cells to that typically observed during inflammation. Activation of cells in the presence of known sialidase inhibitors such as exogenous sialic acid prevents the desialylation and lowers cell adherence.
Interestingly, it has been shown that cell adherence is important in several infectious processes. Of greatest interest has been the effect of cell-to-cell transmission of human immunodeficiency virus (HIV). Infection of mammalian cells by HIV is known to be facilitated by activation of its cellular target.
The critical events in the multistep process of cellular activation that facilitates infection with HIV have not been identified. It has been shown that increased expression of endogenous sialidase follows activation of T lymphocytes by lectins and it has been suggested that this increase may play a role in the differentiation and maturation of these cells.
Specific inhibitors of sialidase activity have been used in vivo in mice to decrease mutual adhesion of blood platelets and to inhibit accumulation of leucocytes in microvascular beds that had been laser-irradiated. (Gorog, et al, Br. J. exp. Path. 61 (1980), 490).
Various sialidase inhibitors have previously been tested as treatment for influenza. Kumar, et al. (Carbohydrate Research, 94 (1981) 123-130) disclosed a method of synthesizing various neuraminic acids. Nohle, et al.(J. Biochem, 126 (1982) 543-548) discloses methods of synthesis of 2-deoxy-2,3-dehydro-N-acetyl-neuraminic acid and 2-deoxy-2,3-dehydroneuraminic acid and discusses the oral administration and secretion of the sialic acids. No method of using these inhibitors for anti-infective or anti-inflammatory use are taught therein. Nagai, et al. (Biochemical and Biophysical Research Communications, Vol 163, No. 1 (1989) and Miyaichi, et al. (Shoyakugaku Zasshi, 42 (3) (1988) 216-219) disclose use of a natural product from the leaf of Scutellaria baicalensis as an inhibitor of mouse liver sialidase, but its application to the treatment of inflammation is not discussed.